Geologic formations are used for many purposes such as hydrocarbon and water production. Boreholes are typically drilled into the earth in order to intersect and access the formations.
Electromagnetic induction resistivity instruments have been used for some time to determine the electrical conductivity of earth formations surrounding a wellbore. More recently, development of deep looking tools based on transient field behavior, which may capable of formation evaluation at distances ranging from tens to hundreds of meters, has been attempted. In transient electromagnetic (TEM) methods, voltage or current pulses that are excited in a transmitter initiate the propagation of an electromagnetic signal in the earth formation. The transmitter and receiver may be at the surface or within the wellbore. Electric currents diffuse outwards from the proximity of the transmitter into the surrounding formation. At different times, information arrives at the measurement sensor predominantly from different investigation depths. Generally, early-time signals predominantly relate to near-zone responses (lesser depths of investigation) and late-time signals predominantly relate to remote-zone responses (greater depths of investigation).